A gas sensor responds to a specific gas contained in other gases such as air and produces an electric signal, a light signal and the like depending on the concentration of the specific gas. Currently, various gas sensors using a detection method wherein chemical properties such as adsorption, reaction, and light emission are often used to identify gas components have been known. As a conversion method into a signal, there is an energy-conversion method, for example using electromotive force, which directly obtains a sensing signal by contacting gases. However, an energy-control method which converts a change in device properties such as material properties, e.g., electrical resistance, transistor properties and the like, into a signal, is mainly used.
As typical materials for gas sensors, oxides having rather difficult reductiveness, such as SnO.sub.2 and ZnO types, are available. Among them, most current commercial devices use a porous sintered body of the SnO.sub.2 type, which is n type semiconductor. As semiconductor gas sensors which use such an oxide semiconductor, there are surface control types in which interactive reaction with a gas stays on the surface and bulk control types in which the reaction extends to a semiconductor itself. Among them, most are the surface control type like sensors of combustible gases- In such surface control type sensors, a chemical reaction occurs on a semiconductor surface. However, the activity is often not enough with a pure semiconductor. Therefore, the sensing function is improved by dispersing powder of a noble metal, a metal oxide and the like on the semiconductor particles.
However, to detect a gas by using these conventional gas sensors, it is required to heat the gas up to a temperature on the order of 200.degree. C. to 300.degree. C. Moreover, with those sensors there is insufficient selectivity of gas species because of difficulty in specifying gas species since these conventional gas sensors may respond to many species of gases. Therefore, further improvement is desired.
The main object of the present invention is to provide sensor materials which are able to sense gases at room temperature, and which also are superior in selectivity of the gases sensed.